Has anyone here used aftermarket K&N air filters?

[Fumesucker]

1. It's been quite a while but I've had motorcycle and car K&N

They are a high quality product and they might possibly give you a slight mpg boost but unless you're planning on keeping the car for a long time you're unlikely to recover your investment.

If your car is a 96 or later and you don't have an electronic MPG readout I'd recommend a ScanGauge for getting the best mileage.

http://www.scangauge.com/

Once you can see when and where you are using the most gas it becomes a game to reduce your gas consumption to the minimum possible.

If you already have a MPG gauge then use it to fine tune your driving style, there's more efficiency gain for less money in driving style than any other single thing you can do to a car.

Seriously, my daughter had an Expedition with the 5.4 and a towing package, she had a hard time breaking 11 mpg with that truck where I could get over 18 mpg over the same trip just by adjusting my driving style to use minimum gas.